The present invention is directed to memory devices for storing digital information, and is more particularly directed to thin film memory devices for storing the information in the form of magnetic states.
Memory devices which enable the user to selectively write, or store, information therein and subsequently read information therefrom are referred to as random access memories (RAMs). One type of RAM that is commonly employed as the main memory in many computers is a dynamic RAM. This type of RAM stores information by means of capacitive memories, in which the voltage stored in a particular capacitor represents a specific bit of information. In order to maintain the voltage levels in the storage capacitors, the memory must be continuously replenished with electrical charges. As such, this type of memory requires a constant source of electrical power to be connected to it, to carry out the replenishing function. If the source of electrical power is disconnected, the charges in the storage capacitors eventually dissipate, and the stored information is lost. This type of memory device is often referred to as a "volatile" memory because of this characteristic.
Although volatile memories have obtained widespread popularity in certain applications such as computers, their need for a constant source of electrical power renders them unsuitable for other applications. For example, in automobiles which employ digital indicators on the dashboard, a non-volatile memory, which does not require a constant source of electrical power, is necessary to store certain information, such as the total mileage reading for the odometer and the like. Furthermore, in other applications, such as military applications, any memory device which is employed must be insensitive to radiation and the like, and be capable of providing a non-destructive readout, in addition to being nonvolatile.
To provide these features, magnetic array memory devices which store the digital information in the form of magnetic states, rather than capacitive charges, have been employed. Basically, a magnetic array memory device comprises a plurality of conductive bit lines which are parallel to one another, and another plurality of conductive word lines which are parallel to one another and orthogonal to the bit lines. A magnetic storage cell is formed by a magnetically coercive material located at the intersection of each word line and each bit line. When currents are passed through a word line and a bit line, a net magnetic field is generated at their intersection. The direction of this magnetic field is stored as a magnetic dipole in the magnetically coercive material at the intersection of the word and bit lines. The direction of this dipole can be subsequently sensed, using inductive techniques, to read the stored bit of information.
In an effort to increase the packing density of magnetic array memories, and thereby increase the amount of information that can be stored per unit area, thin film technologies have been employed in the fabrication of these memories. Basically, a thin film magnetic array memory comprises an insulative substrate, e.g., silicon, having the bit lines deposited thereon. Each bit line consists of a thin metal conductor, such as aluminum. The intersecting word lines, which also comprise thin metal conductors, are overlaid upon the bit lines. At the intersection of each word line and bit line, a thin film of magnetically coercive material, preferably permalloy, is interposed between the word lines and bit lines. This magnetically coercive material forms the magnetic memory cells in which bits of information are stored.
Thin film magnetic array memories offer considerable advantages over previous magnetic array memory technologies, both in terms of cost to manufacture and packing density. However, the structure of the thin film array presents practical limitations on the packing densities that can be achieved. More particularly, by the very nature of thin film technology, the word lines are located very close to the bit lines. As efforts are made to increase packing densities the word lines also become located closer to one another. As a result, any given word line is located relatively close to the memory cell defined by the intersection of an adjacent word line and a bit line, and the magnetic field generated around the word line of interest could adversely affect the information stored in adjacent memory cells. Specifically, when current is passed through the word line to read data from the memory, the magnetic field created by this current causes the edge domains of the magnetic states stored in adjacent memory cells to expand, or "creep", in the direction of the bit lines, under the influence of dynamic orthogonal fields from the word lines. Consequently, the density of the magnetic domain stored at the immediate intersection of a word line and a bit line gradually decreases, with a resulting decrease in signal level that can be obtained when inductively reading the stored state. After multiple read operations, the signal level may have decreased to the point where it is not possible to reliably detect the bits stored in the memory.
One type of attempt to eliminate this problem of domain creep has been to magnetically shield the magnetic fields generated by the word lines from one another. In this approach, a magnetic shielding material, such as permalloy, is placed around each word line to prevent the field generated by a word line from penetrating to other word lines. This approach is not without attendant limitations, however. For example, in order to effectively shield the magnetic fields, the magnetic shielding material has to be relatively thick. The requirement for thick films around each word line adds to the overall bulk of the memory device, and again imposes a practical limitation on the density with which the word lines can be packed adjacent one another. In addition, the magnetic shielding film must be removed from the bit lines so as not to adversely affect the operation of the memory device. This need to remove the film from certain portions of the device adds to the number of processing steps that are required, and therefore increases the overall cost of the device.
Accordingly, it is desirable to provide a thin film magnetic array memory that offers increased packing density without the need to employ a thick magnetic shielding film around the word lines to prevent magnetic domain creep.